1. Field of the Invention
The present invention relates to the field of semiconductor fabrication, and more specifically to a method for repeatable plasma ignition and RF tuning in plasma systems.
2. Background Information
In the fabrication of semiconductor devices, processing in a plasma is commonly used in many process steps, such as sputter deposition, sputter etching, and other plasma or plasma assisted processes such as plasma enhanced chemical vapor deposition, reactive ion etching (RIE), and other plasma processes.
Typically, the plasma is generated by coupling radio frequency (RF) electro-magnetic energy to the plasma. The RF energy is supplied by an RF generator coupled to a power supply. Because the plasma has a variable impedance, a matching network is employed to match the impedance of the power supply with that of the plasma. The matching network may include one or more capacitors and one or more inductors to achieve the match and thereby tune the RF power. Typically, the tuning may be done automatically by an automatic matching network (AMN). When tuned, most of the power output of the RF generator is coupled to the plasma. The power to the plasma is often referred to as forward power. When the system is improperly tuned, some of the power is not coupled to the plasma, but is lost, for example, as reflected power. When power is lost in this manner, the plasma is unstable, such that the characteristics of the plasma vary depending upon the degree of tuning. An unstable plasma is problematic in that it is difficult to achieve consistent results from the plasma process, since the characteristics of the plasma vary during the process. For example, if an etch recipe designed to remove a given amount of material is performed without proper tuning, such that the reflected power is high during a significant portion of the etch, a reduced amount of etching will occur compared with an etch performed with proper tuning for most of the etch. Additionally, other characteristics of the etch, such as uniformity, etc. may be adversely affected by an unstable plasma.
During tuning, the matching network attempts to achieve a match through an iterative process during which a variable capacitance and/or a variable inductance is changed. Often, a match cannot be achieved, and the reflected power may not be reduced to an acceptable level by the matching network. At the beginning of the plasma process, the matching network has a small range of, for example, capacitance values over which a plasma will be ignited. If the reflected power does not reach an acceptable level in this range, the plasma process cannot start. Even if a plasma has been struck, the reflected power may reach unacceptable levels during the process, resulting in the unstable plasma condition described above. For example, FIG. 1 illustrates an example of reflected power as a function of time, as shown by curve 101, for a condition where there is difficulty in achieving a match. As can be seen, oscillations in the reflected power occur during tuning. These oscillations may result in the average reflected power increasing, rather than decreasing during tuning. It is particularly difficult to tune the power at the beginning of the plasma process, because voltage transients occur which may be larger than the voltage rating of the blocking capacitor used in the RF tuning circuit. When the rating is exceeded, an arc occurs inside the capacitor, creating an unstable plasma condition. Once the arc has occurred, the power must be turned off and the process repeated. When the power is turned on again, the same cycle may again occur.
It is typically difficult to ignite and stabilize a plasma under high DC bias voltage conditions, which typically occur under low pressure conditions and/or under high power conditions. It is desirable to form a plasma under these conditions, as better performance, for example, of an etch with regard to characteristics such as selectivity, process window, aspect ratio dependency, minimum feature size, reduction of undercutting, and other characteristics, is achieved under these conditions. While some plasma system manufactures are striving to produce systems which may operate in the low pressure and/or high power regime, such systems are not widely available for production use, and are very costly. In addition, there exists a large installed base of systems for which desired conditions are outside of the normal operating range. Alternatively, attempts can be made to strike and tune a plasma under these desired conditions. However, poor repeatability results for the reasons described above. As a further alternative, it may be possible to strike and tune the plasma under high power/high pressure conditions, with a gas additive such as C.sub.2 F.sub.6, tune at these conditions, and then drop the pressure to a desired level while continuing to tune. However, this method also leads to poor repeatability under high voltage conditions. Additionally, while certain gases lead to more stable plasmas, it may not be desirable to have these gases from a process standpoint.
What is needed is a method for repeatable plasma ignition and RF tuning. The method should allow for plasma processing under high DC bias voltage conditions, such as in the low pressure and/or high power regime. The method and apparatus should be capable of extending the operating range into the high bias voltage regime of existing systems. Further, the method and apparatus should be capable of extending the operating ranges of advanced systems as well.